🔍 Key Concepts

• Pulse Width Modulation (PWM) creates an "average" AC waveform from a fixed DC bus by changing how long each pulse is ON vs OFF • In a variable speed drive, the DC link voltage feeding the IGBT inverter bridge is essentially constant in magnitude • The figure shows pulses that sometimes connect the motor phase to the + DC bus and sometimes to the – DC bus (change in polarity)

💭 Think About

• Look at the height of each pulse in figure B: does the pulse PEAK level change, or does only the ON-time (width) change? • Compare the positive and negative half cycles in the drawing: does the output ever reverse direction (polarity), or does it always stay on one side of zero? • Ask yourself: in a PWM inverter using a DC link, which is easier and typical—changing the DC bus magnitude, or changing how long the switches are turned on and which side of the DC bus they connect to?

✅ Before You Answer

• Verify whether the pulse amplitude (voltage level from the DC link) remains the same for all pulses in a given phase • Verify whether pulse width increases and decreases following the red sine reference in the illustration • Verify whether the output phase is sometimes tied to the positive DC bus and sometimes to the negative DC bus, indicating a change in polarity